The present invention relates in general to an electrochemical device, and more particularly to improvements in an electrochemical device of laminar structure which includes an electrochemical cell using a planar solid electrolyte body.
There have been known various electrochemical devices using solid electrolyte, for example, oxygen sensors to detect the oxygen concentration of an exhaust gas emitted from internal combustion engines of automotive vehicles. The typical examples of such oxygen sensors include an oxygen sensor which comprises a body of oxygen-ion conductive solid electrolyte such as zirconia ceramics and which operates to determine the oxygen concentration according to the principle of an oxygen concentration cell. Also known in the art are electrochemical devices such as sensing and pumping elements for detecting hydrogen, nitrogen, carbon dioxide, etc. In such electrochemical devices, solid electrolyte materials have been generally used in the form of a tubular body which has an elongate bore closed at its one end. In recent years, however, it has been attempted to replace the tubular solid electrolyte body with a solid electrolyte body of planar shape, as disclosed in U.S. Pat. Nos.: 4,334,974; 4,282,080; and 4,300,990, in view of relatively low productivity and high cost of manufacture of solid electrolyte bodies of tubular type, and from the standpoint of easy assembling of parts with a planar solid electrolyte body. When such planar solid electrolyte bodies are employed, suitable electrodes are disposed on the surfaces of the planar body or layer of solid electrolyte, and the electrolyte bodies and other parts are assembled in stack into a laminar structure constituting an electrochemical cell or sensing element.
As indicated above, such an electrochemical cell of laminar structure is usually constituted by a planar solid electrolyte body and two (a pair of) electrodes. This laminar structure is provided with an internal cavity or chamber into which a gas to be measured (hereinafter referred to as "measurement gas") such as an exhaust gas is introduced. The laminar structure is further provided with an internal space, more specifically, a reference-gas passage in which is introduced a reference substance used as a reference for the measurement gas for measuring an electromotive force induced between the two electrodes. These cavity and reference-gas space or passage are formed so as to extend parallel to the surfaces of the electrodes, but disposed in spaced-apart relation with each other in the direction of the thickness of the cell (in the direction of lamination of the device), and therefore increase the thickness of the cell.
A study by the inventors revealed that an electrochemical sensor using such a thick cell tends to have a relatively large temperature gradient in the direction of thickness, particularly when its solid electrolyte body is heated, by a suitable heater, to an elevated temperature for accurate and reliable operation of the cell, and consequently the solid electrolyte body is likely to be damaged due to thermal stress caused by such a large temperature gradient. In addition to this problem, the inventors found that the large thickness of the cell is a potential factor for an increase in time required for the solid electrolyte body to be heated to its operating temperature. Although electrochemical cells of planar laminar type are advantageous for its relatively compact structure, as compared with a conventional cell of tubular type, there is a growing need of minimizing the size of the planar electrochemical cells, to meet the recent trend in this field of industry that the electrochemical cells are increasingly employed for automotive vehicles as oxygen sensors for sensing oxygen in exhaust gases from the engines.
As indicated above, it is usually preferred that the electrodes and solid electrolyte body constituting an electrochemical cell of the electrochemical device are heated by a suitable heater to an elevated temperature in order to assure an efficient, accurate operation of the electrochemical cell while the temperature of a gas to be measured is relatively low. For this reason, a known electrochemical device is provided with a heater, i.e., a heating layer, which is disposed adjacent to an electrochemical cell consisting of a solid electrolyte body and electrodes, such that an insulating layer is interposed between the heater and the electrochemical cell. Alternatively, a heater is disposed at a position spaced from the electrodes in the same plane of the electrochemical cell.
By the way, one example of electrochemical devices of laminar structure as indicated above, includes an electrochemical pumping cell which has two porous electrodes disposed on opposite surfaces of a porous solid electrolyte layer, and an electrochemical sensing cell which has two porous electrodes disposed on another solid electrolyte layer. These pumping and sensing cells are assembled in stack into an electrochemical device of laminar structure. This kind of device is known to be useful as a sensor having a wide measurement range. In the case where such an electrochemical device is provided with a heater as previously indicated, the heater is generally disposed on the side of the sensing cell. If a heater is disposed on the side of the pumping cell, a pattern of heating element located over the pumping electrodes will tend to excessively heat the electrodes and cause local unevenness in diffusion of a measurement gas through the pumping cell, and may consequently reduce the accuracy of measurement of the device. In the case where the heating element is not disposed over the electrodes, but located sideways of the electrodes to heat them in the lateral directions, the temperature of the heated electrodes is liable to be varied from one area to another, causing local unevenness in amount of flow of a pumping current through the cell. This may also be a cause for lowering the measuring accuracy of the electrochemical device.
However, when a heater is disposed on the side of an electrochemical sensing cell, this sensing cell is interposed between the heating element and the electrodes of a pumping cell. The interposition of the sensing cell permits better distribution of heat over the pumping cell. Further, this arrangement will not hinder the diffusion of the measurement gas through the pumping cell. On the other hand, the interposition of the sensing cell between the pumping cell and the heater inherently results in insufficient heating of the pumping cell by the heater, and therefore the use of the heater does not necessarily contribute to improvement in the pumping ability of the pumping cell. In the light of this disadvantage, it is considered to use a heater having a larger heating capacity for generation of an increased amount of heat. Such a solution, however, may lead to shortening of the service life of the heating layer of the electrochemical device.